WALNUT CREEK — The motive that drove a man who shot and killed a California Highway Patrol officer in September remains a mystery to investigators, despite a new report that chronicles increasingly erratic behavior by the gunman in the years leading up to the shooting.

The report from the Contra Costa County Sheriff’s Office seeks to shed light on the murder of CHP Officer Kenyon Youngstrom, 37, during a traffic stop in September 2012. The gunman who shot Youngstrom, Corning resident Christopher Boone Lacy, was shot and killed seconds later by Youngstrom’s partner, Tyler Carlton.

The investigative report reviewed an already known history of Lacy’s erratic behavior, from a breakdown in college to a bizarre encounter with a police officer during a 2006 DUI arrest in Sausalito.

The report also showed that in the year leading to the fatal traffic stop, Lacy, 36, began to withdraw, becoming distant from family and friends. Lacy’s computer showed that he had researched the sovereign citizen movement, a loose coalition of people who reject federal, state and local laws.

“Although we can develop a better picture of Lacy, in the end, we will never know exactly why he killed Officer Youngstrom,” said sheriff’s office Sgt. Jose Beltran.

CHP Sgt. Diana McDermott, speaking on behalf of the Youngstrom family, expressed “deep appreciation” to the sheriff’s office and the Contra Costa District Attorney’s Office for putting in extra hours in the six months investigating the case since Youngstrom was killed.

“They put many, many hours into this, and the investigation speaks for itself,” McDermott said. “It shows the kind of person (Lacy) was.”

On Sept. 4, Youngstrom was removing a dead deer on the shoulder of Interstate 680 in Alamo when CHP Officer Carlton pulled Lacy over for an obstructed license plate. Youngstrom, who was parked directly in front of Carlton, walked over to Lacy’s car to assist in the traffic stop.

As Youngstrom approached the driver’s side window, Lacy pulled out a handgun without warning and shot Youngstrom in the neck. Carlton immediately drew his gun and shot Lacy, hitting him several times and killing him.

Sheriff’s investigators said Lacy was increasingly a “loner” in the year leading to the shooting. He worked temporary jobs in the Silicon Valley and split time between a trailer in Corning and a rented room in Sunnyvale.

Investigators also said they found a large number of documents on Lacy’s computer about the sovereign citizen movement. Though Lacy never declared himself a sovereign citizen, officials believe he subscribed to their viewpoints, Beltran said in the summary of the report.

A 2010 FBI report describes the sovereign citizen movement as “extremist,” a domestic terror threat group whose members are responsible for a variety of violent and financial crimes. Adherents believe that though they live in the United States, they are separate or “sovereign,” and they do not answer to any government, according to the FBI.

When a police officer in Sausalito stopped Lacy in 2006 on suspicion of driving under the influence, Lacy handed the officer a “bill of rights” card and refused to answer any questions. Though Lacy was arrested, no charges were filed.

Other items found on Lacy’s computer, when it was decrypted, included a “wish-list” that including having “mud on a license plate,” and an obstructed plate is what led to the traffic stop the day Youngstrom was shot.

Investigators also reported that the handgun Lacy used in the shooting was legally purchased in 2010 and registered to him, though he did not hold a concealed-weapons permit. Registration for Lacy’s Jeep Wrangler had expired in 2011.

“There was no indication before the shooting that Christopher Lacy was going to kill or assault law enforcement officials,” Beltran said in the report.

Youngstrom, a seven-year CHP veteran, died Sept. 5 after he was taken off life support at John Muir Hospital in Walnut Creek. His liver, pancreas, heart and kidneys were donated to four patients.

A Cordelia resident, Youngstrom was the 223rd CHP officer to be killed in the line of duty since 1929.

WASHINGTON – Hillary Clinton challenged Congress on Thursday to combat fake and misleading news on social media, using a post-election appearance to tackle an issue that gripped her presidential campaign and culminated with a shooting incident Sunday in Northwest Washington.